The pixels of one and the same column are linked to a common column conductor which is in turn linked to a respective reading circuit corresponding to this column. The pixels are addressed by a row conductor and, when reading an addressed pixel, a potential representing the electrical charges generated by the lighting of the pixel is applied to the column conductor and transmitted to the reading circuit.
More often than not, the reading is done by double-sampling, that is to say that two successive values of the potential of the column conductor are sampled and the difference in values is measured; one of the values corresponds to the potential taken by the conductor when resetting the pixel after a charge integration phase; the other corresponds to the potential taken because of the charges produced by the lighting of the pixel. The sampling of the column potential is done in a sampling capacitor (or two sampling capacitors).
One way of producing the reading circuit consists in using an analogue-digital convertor of the ramp converter type to directly produce a digital output signal.
The general principle of a ramp analogue-digital converter is as follows: to convert a potential level present on a column conductor, this conductor is linked to a first input of a comparator, and a linear voltage ramp of known slope is applied to a second input of the comparator. The voltage ramp starts, at an instant 0, from a predetermined reference voltage level; a counter counting at a fixed frequency is initiated at the same instant 0. When the voltage level of the second input of the comparator reaches the voltage level imposed on the first input, the comparator switches over; the switching over of the comparator initiates the storage in memory of the content of the counter at the instant of the switchover; this digital content therefore represents the time taken by the ramp of known slope to pass from a reference level to the level present on the column conductor. It therefore represents a digital value of the potential present on this conductor.
For a precision conversion, for example on 14 bits, a long ramp duration is needed (and therefore a low image output rate), or a count at very high frequency is needed; but then, the switchover time of the comparator may introduce an error on the result of the count.
In a matrix-array that comprises as many comparators as there are columns of pixels, there is a risk of having switchover times that are not identical from one comparator to another. This spread of delays creates a fixed pattern noise (FPN) in the image detected, because the systematic counting error is different for each column.
For example, the period of the counter, which corresponds to a least significant bit in the conversion, is approximately 3 ns (frequency 300 MHz); the switchover time can be approximately 300 nanoseconds with a dispersion of 2% between the different comparators, or 2LSB. The fixed noise that results therefrom can be seen in the reproduced image.
This noise could be eliminated by using a single converter for the entire matrix-array, but it would have to work extremely rapidly and would be that much more sensitive to temporal noise; furthermore, it would be necessary to add a sophisticated multiplexor to it which in turn would introduce other sources of fixed noise. It is therefore preferred to use a convertor for each column.